Ship's hulls are very large and are complexly contoured in both the vertical and longitudinal directions. The world's population of ships has a very significant number of different sizes and shapes.
Coating of the exteriors of ships requires using abrasive blasters for surface preparation and painters for application of paint. Both blasters and painters must be brought into close proximity to the portion of the hull they are working. Neither blasters nor painters can perform their work on much more than 75 square feet of hull surface without moving or being moved to another location.
In earlier times, worker movement from place to place around a ship's hull was accommodated by building staging around the ship.
More recently, this movement has been accomplished through the use of manlifts. A conventional manlift includes a staging basket mounted on an arm which has the capability of being hydraulically lifted, extended and rotated; this arm being mounted on a carriage powered by an internal combustion engine. The carriage has the capability of being moved from place to place on a horizontal surface.
Even more recently for abrasive blasting, efforts have been made to replace the worker in the manlift basket, with an enclosed shotblast head which has the capability of catching, processing and reusing the abrasive. However, this approach has had little acceptance because of the cost to purchase and operate the apparatus, plus operating difficulties with the devices actually available.
Since ships are very large vessels which operate on large bodies of water, their construction and repair by dry-docking almost always takes place immediately adjacent to large bodies of water.
Pollution of these large bodies of water including Great Lakes, rivers, seas, bays and oceans has become a much greater concern to societies around the world because of the negative effect of this pollution on the vegetable and animal life which depend upon these bodies of water. This concern has grown as more of the public elects to use these bodies of water for recreation through swimming and boating as well as living adjacent to them in hotels, houses, apartments and condominiums.
Abrasive blasting of a ship's hull necessarily creates a significant quantity of particulate material, usually dust comprised in part of smaller particles of the abrasive medium as it breaks down upon being propelled pneumatically against the ship's hull and in part of small particles of the ship's paint and steel which is removed by the abrasive. While this dust is not currently officially considered to be hazardous, it is nevertheless noxious to the public and does contain toxins in apparently nonhazardous quantities.
Because a portion of this dust inevitably is blown over the adjacent body of water, small quantities of these toxins find their way into the water. Further, if the large percentage of the spent abrasive which lands on the dry dock floor is not promptly cleaned up, trace amounts of the toxins leach out during rainstorms or from other sources of water used in ship repair and are deposited into the body of water from the dry dock's drainage system. Toxic petroleum products including fuels, lubricants and greases associated with manlift operations can similarly be carried through the dry-dock drainage system into the adjacent body of water.
Typically, a ship has a large quantity of exterior mechanical equipment. This equipment, which is expensive to repair and purchase, is subject to severe damage if infiltrated by the dust from abrasive blasting, which is itself very abrasive. This mechanical equipment, which includes interior ventilation systems, must be temporarily covered with protective covering during abrasive blasting. This temporary covering prevents the interior ventilation systems from being operated or repaired when abrasive blasting is underway.
Virtually all the equipment required for abrasive blasting has mechanical components. This includes air compressors, manlifts, forklifts, dust collectors and dry-dock cranes. Since this equipment must operate during abrasive blasting, it cannot be protected. It therefore experiences very high maintenance cost, extensive out-of-service periods, and shortened operating life.
Coatings on dry-dock horizontal surfaces experience short lives as they are abraded off by the combination of spent abrasive and vehicular and personnel movement, including that which accompanies shoveling and sweeping.
Workers who are free to proceed with exterior ship construction and/or repair tasks which do not involve mechanical ship's components are disrupted, made less efficient and exposed to respiratory and eye aggravation when abrasive blasting is proceeding concurrently. Workers and ship's personnel transiting through the abrasive dust cloud to and from the interior of the ship are similarly affected.
Most ships operate in a corrosive saltwater/spray environment. Therefore, the most popular marine paints are solvent-based vinyls and epoxies. Some marine paints contain zinc or cooper. During the time that these paints are being applied, overspray is often blown into the adjacent body of water. This same overspray can coat itself on nearby boats, buildings, waterside cafes and cars, causing expensive damage and infuriating the public. Even the portion of the overspray which lands on the dry-dock floor can find its way back into the adjacent body of water as it attaches itself to dust or dirt particles on the floor of the dry dock which are washed by water through the dry dock's drainage system.
Nonwater-based paint solvents common in marine coatings release volatile organic compounds (VOCs) into the atmosphere during the time that they are evaporating, during the paint curing process. Regulatory authorities are becoming increasingly concerned that these VOCs are damaging the environment. While VOC emissions from marine paints may not be apparent to the public, they are a matter of growing regulatory oversight, and likely will ultimately have to be reduced. The only current way to dispose of these invisible VOCs is to contain the air into which they are released, and then process that air through a VOC incinerator.
Best management practices being currently utilized to minimize the amount of abrasive dust and paint overspray being blown beyond the dry-dock perimeter include placing a curtain over each end of the dry dock, performing abrasive blasting downward only, using airless paint spray equipment, and ceasing operations when wind velocities become higher than a predetermined limit. However, these practices nevertheless permit a significant percentage of the airborne abrasive dust and paint overspray to blow to outside of the perimeter of the dry dock. In addition, these practices do nothing to reduce the many other negative affects of the ship coating process.
Recently, some shipyards have begun shrouding ships, from the weather deck down to the dry-dock structure, with very large strips of material. This material must be somewhat porous to keep it from shredding in the wind. However, the lives of these large strips of material are short because of damage from wind, handling, errant abrasive blasting and other hazards inherent to the heavy industrial environment prevalent in shipyards. Because of the basic cost of the shrouding material itself, its short life in the shipyard environment, the cost of installing, removing, handling and storing it, this approach is very expensive. While this approach contains even more airborne abrasive dust and paint overspray within the dry-dock perimeter than currently accepted best management practices, some still escapes through the necessarily porous material and through the joints where the strips of material overlap. In addition, this approach does little to solve the many other negative effects of the ship coating process.
One other existing technology exists that reduces dust from sandblasting, that is the technology of vacuum blasting. However, this process is very slow and very costly, from an equipment and manpower standpoint.
With regard to approaches to resolve the many problems associated with the coating of ships, as expensive as the coating process is or may become, the major cost consideration is the speed with which a ship may be coated or recoated. This is because of the daily amortization and operation costs of the dry dock required to lift the ship out of the water for recoating ($5,000 to $20,000 U.S. per day) and the ship itself which is out of service during recoating ($10,000 to $100,000 U.S. per day). These costs demand that with whatever solutions are developed to solve the existing problems with abrasive blasting and coating of ships, elapsed time of the coating process be of the essence.